Medical Device Accelerated Aging

In the medical device registration process, one important task during the research and development stage is determining the lifespan and validity period of the medical device. Among the common methods for determining the lifespan and validity period of medical devices, natural aging tests and artificial accelerated aging tests are the two most widely used approaches. This article will explain what a medical device accelerated aging test is.

What is an Accelerated Aging Test?

An accelerated aging test is a method where artificial conditions are created indoors or within a device to simulate atmospheric or specific environmental conditions. These factors are intensified in order to obtain experimental results in a short period.

The purpose of an accelerated aging test is to provide a relatively fast method for measuring the degree of change in material properties over long-term use. If the initial accelerated methods do not result in aging effects similar to those that occur during actual usage, or if no mechanisms are observed in long-term experiments, the accelerated test should be re-evaluated and improved after data is obtained during the problem identification and pre-experiment analysis stages. The choice of the experimental method depends on the materials being tested, their intended applications, damage modes, and financial resources. Therefore, most international standards use this method to assess the aging resistance of materials.

The level of acceleration in an accelerated environmental test is typically expressed by an acceleration factor. The acceleration factor refers to the ratio of the device’s lifespan under normal operational stress to its lifespan under accelerated environmental conditions. In simpler terms, it indicates how long one hour of testing corresponds to normal usage time. Hence, calculating the acceleration factor becomes a core issue in accelerated life testing. Accelerated aging tests are essential for medical device manufacturers to obtain critical performance and validity period data for new products.

The fundamental principle of accelerated aging tests is the same for active and passive products: both rely on intensified stress environments for reliability testing. The difference lies in the aging methods and the calculation formulas used.

Important Considerations

It is crucial to note that any accelerated aging test method carries risks if real-time or atmospheric test results are not available. Therefore, the data provided by the designed test methods should ultimately meet the product’s standard requirements.

Medical Device Accelerated Aging Testing Standards

- ISO 11607-1: Standard for final sterilization of medical device packaging, including basic principles for accelerated aging testing.

- IEC 62506: Standard for guiding accelerated life tests for active medical devices.

- EU Medical Device Regulation (MDR) (Regulation (EU) 2017/745): While no specific testing methods are provided, it sets forth the compliance framework and principles for accelerated aging testing.

- ASTM F1980: A guideline commonly referenced in the EU for accelerated aging of sterile barrier systems and medical devices.

Artificial Accelerated Aging Test Methods

Artificial accelerated aging test methods primarily include: weathering tests, thermal aging tests (such as oxygen deprivation, hot air oxidation, oxygen absorption, etc.), humid heat aging tests, ozone aging tests, salt spray corrosion tests, cold resistance tests, and anti-mold tests.

01. Weathering Test

In natural environments, the normal service life of materials is referred to as weather resistance. Methods for evaluating the service life of polymer materials in natural environments include outdoor aging tests and artificial aging tests. Outdoor aging tests are the most suitable method for evaluating material practicality. However, the aging of polymer materials is influenced by a combination of factors like heat, light, mechanical friction, chemicals, and microorganisms, with factors such as sunlight exposure and rain being difficult to control, which leads to longer testing periods. Accelerated aging tests simulating outdoor climate conditions in the laboratory are an important approach for weathering tests. Typically, weathering tests use weathering test chambers that employ carbon arc lamps, xenon lamps, or ultraviolet fluorescent lamps to simulate sunlight UV radiation, periodically spraying salt solution onto the samples to simulate rain and salt particle effects. The combined action of multiple environmental factors forms the experimental process.

02. Thermal Aging Test

Heat is one of the primary factors that promotes aging reactions in polymers. Heat can cause molecular chain breakage, generating free radicals that initiate chain reactions, leading to polymer degradation and crosslinking, which degrade material properties. The thermal aging test accelerates the aging process of materials under the influence of oxygen and heat, reflecting the material's resistance to thermal oxidation aging. The experimental temperature is determined based on material requirements and testing objectives. The upper limit of the temperature is typically determined by technical specifications: for thermoplastic materials, it should be below their Vicat softening point; for thermosetting materials, below their heat deformation temperature, or by exploratory experiments to select a temperature that does not cause decomposition or obvious deformation of the sample. Common test methods include plastic hot air exposure tests, sulfurized rubber or thermoplastic rubber hot air accelerated aging tests, and heat resistance testing for coatings.

03. Humid Heat Aging Test

In the atmospheric environment, temperature (heat) and humidity (moisture) are inherent factors. Some polymer materials are stored, transported, or used in high-temperature, high-humidity environments. Therefore, the humid heat aging test is of practical significance and economic value. Water vapor at high temperatures has a certain permeability to polymer materials, and under heat, this permeability is enhanced, allowing moisture to penetrate the material's internal structure and accumulate, forming blisters. This reduces the molecular interactions and leads to material aging. Humid heat aging tests typically use humidity test chambers that provide a standard, pollution-free atmospheric environment (gases such as N2, O2, CO2, and water vapor) at temperatures of 40-60°C and relative humidity above 90% RH.

04. Ozone Aging Test

Ozone, although present in small amounts in the atmosphere, is a major factor in rubber cracking. The ozone aging test simulates and intensifies atmospheric ozone conditions to study its effects on rubber, enabling rapid evaluation of rubber's resistance to ozone aging and the effectiveness of anti-ozone agents. Effective anti-aging measures can then be implemented to enhance the service life of rubber products. Waterproof rubber materials and polymer waterproof materials require this type of test.

05. Salt Spray Corrosion Test

When salt particles settle on the surface of a material, they quickly absorb moisture and dissolve into chloride solutions. Under certain temperature and humidity conditions, chloride ions in the solution gradually penetrate the material through micro-pores, causing aging or corrosion of metals. The salt spray test is used to assess a material's resistance to electrochemical corrosion.

06. Cold Resistance Test

The cold resistance of a polymer refers to its ability to withstand performance changes at low temperatures. However, when the environmental temperature reaches a certain low point, the polymer becomes brittle. Low-temperature storage tests can identify a material's characteristics for low-temperature storage. Cold resistance is related to polymer chain mobility, intermolecular forces, and chain flexibility. Saturated polymers with flexible main chains tend to have better cold resistance. In contrast, polymers with rigid substituents or heavy cross-linking have poorer cold resistance.

07. Anti-Mold Test

Mold is a microorganism, and its metabolic byproducts (organic acids) can lead to material failure. To assess the degree of mold growth on materials, artificial anti-mold tests are typically conducted. Common mold strains used in testing include Aspergillus niger, Aspergillus flavus, Penicillium, and Trichoderma. Since different materials are attacked by different mold species, the appropriate strain should be selected based on the polymer material being tested. The test cycle is typically 28 days. A common test method is using a mold aging test chamber, where fungi are cultivated under specific temperature and humidity conditions to evaluate the material’s resistance to microbial aging.